Laser processing of a multi-phase transparent material, and multi-phase composite material

ABSTRACT

A method provides for producing modifications in or on a transparent workpiece using a laser processing device. The laser processing device has a short pulse or ultrashort pulse laser that emits laser radiation having a wavelength in the transparency range of the workpiece and which has a beam-shaping optical unit for beam shaping for focusing the laser radiation. The transparent workpiece is composed of a material that has a plurality of phases, of which at least two phases have different dielectric constants, of which in turn the one phase is a phase embedded in the form of particles, which phase is substantially surrounded by the other phase, and wherein the product of the volume of the particles specified in cubic nanometers and the ratio of the absolute value of the difference of the two different dielectric constants to the dielectric constant of the surrounding phase is greater than 500.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 15/858,705,entitled “LASER PROCESSING OF A MULTI-PHASE TRANSPARENT MATERIAL, ANDMULTI-PHASE COMPOSITE MATERIAL”, filed Dec. 29, 2017, which isincorporated herein by reference. U.S. patent application Ser. No.15/858,705 is a continuation of PCT application No. PCT/EP2016/060742,entitled “LASER PROCESSING OF A MULTI-PHASE TRANSPARENT MATERIAL, ANDMULTI-PHASE COMPOSITE MATERIAL”, filed Jun. 28, 2016, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for efficiently producingmodifications in or on a multi-phase transparent material, in particularusing a short pulse or ultrashort pulse laser.

2. Description of the Related Art

A method for efficiently producing modifications in or on a multi-phasetransparent material, can be used for efficient local modification ofthe workpiece or its material properties, and the composite material canas well be locally modified correspondingly in efficient manner. Inparticular, such a method can be used for efficiently producing a seriesof aligned linear modifications along a predetermined contour, and suchlinear modifications can as well be produced in the composite materialin efficient manner. In this case, the method is suitable for separatingthe workpiece, and the composite material can as well be easilyseparated correspondingly.

Generally, there are many different possibilities for producingmodifications in or on transparent materials using laser radiation.

JP002011147943A describes the suppression of thermal effects in thelaser processing of substrates, for example. For this purpose, the endportion of a substrate is laser-processed by means of a Bessel beam.

For the special case of linear modifications, i.e. for the case ofintentionally producing linear or filamentary defects, a number oftechniques are already available as well.

“Femtosecond filamentation in transparent media”, A. Couairon, A.Mysyrowicz, Physics Reports Vol. 441, Issues 2-4, March 2007, pages47-189, describes the essential mechanisms in the formation offilamentation of ultrashort laser pulses in various transparent media.

It is also known from WO 2012/006736 A2 that by utilizing nonlinearoptical properties of transparent materials (i.a. the electro-opticalKerr effect and multiphoton absorption) it is possible to producemodifications in the glass in form of permanent linear or filamentarydefects or damages by injecting laser radiation of sufficient radiationintensity (>10¹² W/m²). A juxtaposition of such linear damages in glasscaused by temporally and spatially changing laser irradiation (e.g.along a contour on the surface of a workpiece) allows to producespatially extended modifications in the workpiece. With a suitableposition and formation, such modifications may even allow for theseparation of transparent substrates.

The high linear radiation intensities required for this purpose can begenerated by using different physical effects: one possibility is theformation of a “real” filament, a waveguide self-created by the laserradiation by Kerr effect and plasma defocusing with a diameter of about1 μm and a certain length (“soliton”); the other possibility is thetargeted deterministic generation of a linear or filamentary intensitydistribution using a beam-shaping optical unit, in which case thediameter of the intensity distribution is even significantly reduced bythe non-linear interaction with the laser radiation.

A filament is a laser beam of high intensity that is extremelyself-narrowing and is propagating very long in comparison to thediameter of the filament and is formed by one or more short orultrashort laser pulses of sufficient power due to non-linearinteraction of the beam with the medium. Often, the resulting permanentlinear modification, such as a change of the refractive index, or thepermanent linear damage zone in the material is referred to as a“filament”.

The related laser technology is also referred to as Ultra Short Pulse(USP) technology or ultrashort pulse laser technology. As a result ofthe local increase of the refractive index due to the Kerr effect, thelaser beam undergoes self-focusing inside the glass, whereby theintensity increases continuously, which in turn can lead to an avalancherelease of electrons until the electron density gets so high at acertain point that the resulting change of the local complexpermittivity of the material leads to a decrease of the refractive indexand to an increase in absorbance. In extreme cases, a plasma explosionoccurs in which the glass may incur irreversible damage around theplasma formation site, caused by the absorbed laser energy. The decreasein refractive index leads to a defocusing of the residual radiation.After the defocusing of the residual beam, a new focusing may occur,which in turn ends in a collapse along with a release of conductionelectrons. This is referred to as a refocusing cycle. Depending on thepulse power, this effect may be repeated several times, as long assufficient energy is available.

The power of the filamentary propagating laser beam decreases along thefilament. Thus, the first plasma spots will absorb the largest amount ofenergy and also produce the greatest damages. Due to the occurringenergy dissipation, the refocusing cycles are only possible over limiteddistances.

Such an introduction of defects into glass and glass ceramics, referredto as filamentation, is also described in DE 10 2012 110 971 A1.

From WO 2014/111385 A1 it is known that such permanent linearmodifications in transparent materials like glass caused by suitableinjection of short and ultrashort pulsed laser radiation can also beproduced by focusing the laser beam, by suitable beam-shaping, not on a(nearly) punctiform focus area but rather on a “focal line” of a certainlength. In contrast to a real filament, in which the highly narrowedlaser beam propagates along the filament line, loses power in theprocess and finally re-expands (with greatly reduced residual power),the irradiation along the line in the case of the formation of a focalline takes place continuously radially with an incidence angle of lessthan 90° in the direction of the optical axis. This can be accomplishedas described in various embodiments of WO 2014/111385 A1, for example bylenses or lens systems in which the focal distance varies with the beamradius, or by using conically ground lenses (axicons) that form pointsources to a line. Also in these cases, permanent modification or damageof the material may occur due to non-linear interaction with the medium,if the beam shaping is appropriate and the beam power is sufficient inthe region of the focal line. A juxtaposition of such linear damages inglass generated by temporally and spatially changing laser irradiation(e.g., along a line on the surface of a workpiece) allows for spatiallyextended modifications in the workpiece also in this case. With anappropriate position and formation, such modifications can allow aseparation of transparent substrates also in this case. Since asignificant part of the laser radiation is suppliedlaterally/radially—and not nearly just in the direction of the opticalaxis in the filament being formed as in the case with realfilamentation—it is possible to produce significantly longer damagelines in dielectric solids such as glass, without excessive attenuationof the laser beam.

KR 2014-0072448 (A) also discloses a device for cutting glass using anaxicon as an optical unit, in which the glass is irradiated with a laserbeam. A beam with a multiple focus is generated using a collimating lenswithin the laser beam and the aforementioned axicon. The glass isarranged within the focal range.

It is furthermore known that it is possible, by “simply” focusingsuitable laser radiation of sufficient power onto or into a medium (i.e.without focusing the beam onto a focal line, without formation of a beampropagating in filamentary manner) to produce spatially localizedmodifications or damages in the medium by non-linear interaction withthe medium. This is utilized for all forms of laser ablation in theprocessing of surfaces, and also for selectively introducing punctiformmodifications inside a transparent material (“laser scribing”), and alsofor spot welding of two superimposed transparent workpieces, e.g. twosheets of glass. It is also possible to modify a plurality of such focalpoints successively, so that a series of adjacent modifications ordamages is provided along a line, like a string of pearls. Such a methodis described in US 2005/173387 A1, for example.

From “Femtosecond laser-induced color change and filamentation inAg⁺-doped silicate glass”, H. Sun et al., Chinese Optics Letters, Vol.7, Issue 4, pp. 329-331 (2009), it is furthermore known that colorchanges and multiple filamentation in glasses can be achieved by laserirradiation.

Structural changes in glass ceramics may as well be reversible. Thiseffect is described in “On the Reversibility of Laser-inducedPhase-structure Modification of Glass-ceramics”, Journal of LaserMicro/Nanoengineering, Vol. 1, No. 2, 2006.

Glass ceramic is a material that includes an amorphous glass phase andat least one crystalline phase. The production process of glass ceramicsstarts with a so-called green glass that is subjected to a specialtemperature treatment, whereby a partial crystallization takes place,which is called ceramization and during which individual finelydispersed crystals are formed in the glassy material. Thistransformation process of the material can be subdivided into nucleationand the subsequent crystal growth at the crystallization nucleus. Fornucleation, impurities are purposely added to the green glass, whichprecipitate when heated and are effective as crystallization seeds. Forceramization, both processes, i.e. the ideally uniformly dispersed anddense nucleation and the crystal growth must be coordinated. This can becontrolled via temperature.

Structural changes of glass ceramics after laser irradiation are knownfrom the article “Structure and properties of glass ceramics after lasertreatment”, Glass and Ceramics, Vol. 56, Issue 5-6 (1999), 144-148.

Essential properties of a glass ceramic are determined by the volumeratio of the amorphous phase to the crystalline phase, but also by thefact that the glass ceramic also includes the initial seeds, whichdiffer in their composition from the growing crystalline phase. Strictlyspeaking, the glass ceramic therefore includes at least three phases.The different phases of the glass ceramic are characterized by adiffering dielectric constant. This has an effect on the speed of lightin the glass ceramic, since the speed of light in a medium depends onthe value of the dielectric constant, inter alia.

It is also known that bubble inclusions in glass can cause plasmaformation when a laser beam hits the glass-air interface. However, thisis rather a disturbing phenomenon.

SUMMARY OF THE INVENTION

The present invention provides a generic method or a generic compositematerial so that material modifications of a sufficient size, strength,or damage are obtained. Therefore, one aspect of the present inventionincludes producing material modifications efficiently.

For example, linear material modifications of a sufficient length may bedesired in order to facilitate subsequent separation (e.g. by applying abending stress) of the workpiece along a line of adjacently introducedmodification lines. In this exemplary case, these linear materialmodifications may be produced by a laser beam propagating in filamentarymanner, but also by a laser beam focused onto a focal line, or else by alinear series of adjacent focal points.

The method provides modifications produced in or on a transparentworkpiece by means of a laser processing device.

The laser processing device that is used includes a short pulse orultrashort pulse laser which emits laser radiation having a wavelengthin the transparency range of the workpiece and which has a beam-shapingoptical unit for beam shaping, in particular for focusing the laserradiation.

In particular, a transparent workpiece is used which is composed of amaterial that has a plurality of phases, of which at least two phaseshave different dielectric constants, of which in turn one phase is aphase embedded in the form of particles that is surrounded or is atleast substantially surrounded by the other phase, and wherein theproduct of the volume of the particles specified in cubic nanometers andthe ratio of the absolute value of the difference of the two differentdielectric constants to the dielectric constant of the surrounding phaseis greater than 500, preferably greater than 1000, more preferablygreater than 2000.

Surprisingly, the method according to the invention enables themodifications to achieve a greater extent in or on the transparentworkpiece than on a workpiece made of the same material which howeverdoes not have a phase embedded in the form of particles.

In the method, a workpiece is used which is composed of a material thathas a plurality of phases. At least two of these phases have differentdielectric constants. One of the two phases is provided in the form ofparticles and is embedded or substantially surrounded by the otherphase. Furthermore, the product of the volume of the regions of theembedded phase specified in cubic nanometers and of the ratio of theabsolute value of the difference of the two different dielectricconstants to the dielectric constant of the surrounding phase is greaterthan 500, preferably greater than 1000, more preferably greater than2000.

This can also be expressed in the way that the product of the ratio ofthe two different dielectric constants reduced by the value of one andthe volume of the particles specified in cubic nanometers is greaterthan five hundred, preferably greater than one thousand, more preferablygreater than two thousand, wherein the ratio of the two differentdielectric constants is greater than one.

The described relationship can also be expressed in the followingformula: (particle volume/nm³)·(|Δε_(r)|/ε_(r))>x with xϵ{500, 1000,2000}, with ε_(r) being the dielectric constant of the one phase,preferably a glassy medium, or the residual glass phase, and |Δε_(r)|being the absolute value of the difference of the dielectric constantsof the glassy medium and the embedded phase.

The mentioned volume of the particles does not refer to the total volumeof all particles but to the volume of one of the particles; inparticular an average is taken into consideration, for example anaverage of at least 50 percent of the particles, or else a median, aswill be described below.

The material of the workpiece may in particular be a glass ceramic, butalso a composite material. The embedded phase may in particular be acrystalline phase. The regions of the embedded phase that aresubstantially surrounded by another phase and which may in particular becrystals grown in glass ceramic are referred to as particles.

It has surprisingly been found that in particular glass ceramics reactsignificantly differently to the impact of high-energy laser pulses thanthe green glass from which the glass ceramic is made.

In particular, it has been found that in such a multi-phase workpiece itis possible, with a certain laser power, to produce a damage or defectthat has a significantly larger extent than in a single-phase workpiecemade of the same material and with the same laser power. Therefore, themethod according to the invention makes it possible to producemodifications of a larger extend in a multiphase workpiece compared tothe processing of a single-phase workpiece, with the same laser power,or to produce modifications of a comparable size in a multiphaseworkpiece with a laser power that is reduced compared to the processingof a single-phase workpiece.

In a further embodiment of the invention, a series of adjacent linearmaterial modifications are produced in a transparent workpiece along aline. For this purpose, the laser radiation as formed by means of thebeam-shaping optical unit, preferably focused, is displaced relative tothe workpiece in order to produce linear modifications in the workpiecealong the direction of displacement. Each linear modification isproduced by at least one laser pulse. It is also possible to apply aplurality of pulses in time succession at one location, or successivelaser pulses can be emitted in the form of bursts.

In case of filamentation of the material, the linear materialmodifications may each constitute a channel of defects of usually atleast 500 micrometer length, which is typically about 1 micrometer indiameter.

The path or the line along which the adjacent linear modifications arealigned may have a rectilinear or curved shape and/or may be angled orhave corners.

Apart from producing linear modifications, the method according to theinvention can also be used to produce modifications on the surface ofthe transparent workpiece. For this purpose, material is removed fromthe surface by means of the laser radiation. This includes the laserablation process, for example. In this case, the method according to theinvention permits to achieve a deeper and/or more extensive materialremoval on the surface.

The method according to the invention can in particular also be used toproduce modifications inside the transparent workpiece. For thispurpose, the parameters of the short pulse or ultrashort pulse laser canbe adjusted so that damages to the surface of the transparent workpieceare avoided.

In other words, the method according to the invention can therefore alsobe used to generate inner markings or inner damages in the transparentworkpiece. With suitable parameters to exclude damages to a surfacethrough which the laser beam enters the workpiece. In particular it ispossible to produce cavities, so-called voids, inside the material. Themethod according to the invention in particular allows to achieve morepronounced inner markings and larger voids.

A composite material that can be produced by the method according to theinvention, in particular in the form of a glass ceramic, or else apolymer material, is distinguished by the fact that the product of thevolume, specified in cubic nanometers, of a region of the at least onesecond phase and the ratio of the dielectric constants of the firstphase and the at least one second phase reduced by the value of one isgreater than five hundred, preferably greater than one thousand, morepreferably greater than two thousand, wherein the dielectric constant ofthe first phase ε_(r1) is greater than or equal to the dielectricconstant of the at least one second phase ε_(r2).

The product IE=(particle volume/nm³)·(|Δε_(r)|/ε_(r)) with IE>500,preferably IE>1000, more preferably IE>2000 is a characteristicparameter for the interfacial effect (IE) described below. In the caseof the composite material according to the invention, the averageweighted by the volume fractions of the corresponding products is usedas the product IE. As a physical cause it can be assumed that the normalcomponent of the electric field intensity E exhibits a jumpdiscontinuity at an interface of a material. The field strengths E₁, E₂behave inversely proportional to the ratio of the dielectric constants.Thus, E₂/E₁=ε_(r1)/ε_(r2) is valid. Therefore, the field strength willdouble with the transition from a dielectric material with ε_(r1)=4 intoone with ε_(r2)=2.

According to the invention, at least one linear material modificationextends inside the composite material, which material modificationconstitutes a channel of defects of at least 500 micrometers in length.

These linear modifications can extend over the entire thickness of theworkpiece, in particular, if the laser processing device is alignedperpendicular to a surface of the workpiece at the location of themodification to be produced. Each linear modification represents achannel of defects substantially in the direction of the optical axis.It is contemplated to produce such modifications adjacently orsuccessively along a line. In a subsequent process, the workpiece canthen be separated along this line which may as well be a curve of anyshape. The line can therefore serve as a separation line.

The series of linear modifications aligned along a predetermined contourwhich corresponds to the desired separation line or later fracture lineis achieved by a relative movement of the suitably shaped, preferablyfocused, pulsed laser radiation with respect to the workpiece.

To produce a modification, in particular linear modifications as part ofan entire separation line, the laser pulse may as well be emitted in theform of a burst. “Burst” means that the actual laser pulse is split intoseveral partial pulses constituting a pulse packet. In this case, theshort pulse or ultrashort pulse laser is thus applied in the so-calledburst mode. The burst frequency which describes the time interval of thepartial pulses of a burst is usually much greater than the repetitionrate of the pulsed laser and can be 50 MHz, for example, compared to arepetition rate of only 1 kHz, for example. For this reason, all laserpartial pulses of a burst are injected into the workpiece at almost thesame location and all contribute to the formation of a singlemodification, preferably a linear modification.

Other important parameters of the short pulse or ultrashort pulse laserare its wavelength, which is in the transparency range of the workpiece,its average power, its repetition rate, its pulse duration and its beamprofile.

The task of the beam-shaping optical unit is to generate an axially andtransversely defined intensity distribution in the dielectric workpieceof a specific thickness.

Preferably, beam shaping systems are used which generate, instead of aperfect punctiform focusing, an intensity distribution that is extendedin axial direction and compact in transversal direction in theworkpiece.

For this purpose, the beam-shaping optical unit of the laser processingdevice may comprise optical elements of different shapes. For example,concave, convex, plano-concave, plano-convex lenses or even concave,convex, plano-concave, plano-convex lenses or axicons are possible. Withaxicons, it is in particular possible to generate or approximate laserbeams whose intensity distribution corresponds to a Bessel beam fromlaser beams whose intensity distribution corresponds to a Gaussian beam.The method according to the invention also permits material processingby means of (approximated) Bessel beams. Alternatively, and preferredbecause of usually less adjustment need, it is possible to use opticalsystems with selective spherical aberration. In the simplest case, thismay be spherical lenses or systems thereof or optical elements withaspheric faces. Furthermore, diffractive holographic elements withconstant or variable properties can also be used. The aim is always toproduce a uniform highest possible intensity along the optical axis overa distance ranging from several 100 μm up to several millimeters, with alowest possible variation of the central full width at half maximum inthe axial direction and a width of a few laser wavelengths.

Moreover, it is usually advantageous to adjust or control the x, y, zposition as a function of time on the workpiece. Furthermore, in thecase of deviations of the surface of the workpiece from a flat surfaceor in the case of curved surfaces such as tubes it may be advantageousto adjust or control the direction of the incident laser radiation withrespect to the local surface normal.

On the other hand, it is also possible to choose the volume of theindividual particles in dependence of the ratio of the dielectricconstants of the embedded phase and the surrounding phase and of thedesired length of the filaments.

For a given ratio of dielectric constants, the parameter of the volumesize of the individual particles of the embedded phase may be a decisivequantity for obtaining a desired length or a desired form of the linearmodifications in the workpiece.

A particularly easy adjustment of the length of the modifications ispossible via the level of the pulse energy.

For example, in case of workpieces made of a multiphase material and bytaking into account the known values of the different dielectricconstants of the two or more phases, it is possible with the methodaccording to the invention to obtain a desired extent or form of themodification, in particular of the length of a linear modification, byadjusting or varying the volume size of the individual particles of anembedded phase. Such an adjustment or variation is made by appropriateselection of the multiphase material.

In one embodiment of the method according to the invention, an opticalelement is used for deflecting the laser light and for successivelymodifying the point of incidence of the laser beam on the workpiece. Theoptical element may comprise at least one rotating mirror configured asa galvanometer scanner, for example.

Preferably, in case it is desired to produce a linear modification, therelative movement between the focused laser radiation and the workpieceis achieved by providing a workpiece table and a displacement device foraligning the focusing or beam-shaping optical unit to the workpiece forproducing the linear modification in the workpiece and thereafter for anincremental relative displacement between the optical unit and theworkpiece table according to the alignment line. However, a workpiecetable may as well be provided for producing other types of modification.

Experiments in conjunction with the invention have surprisingly shownthat glass ceramics react significantly different to the describedprocess than the green glass corresponding to the respective glassceramic. This difference is particularly evident in the distinctness,extent, or size of the resulting modification, which have been found tobe significantly more pronounced, more extended or larger in the glassceramic than in the green glass. In case of linear modifications, inparticular the length of the resulting linear modifications isincreased. It has been found that this is due to the multiphase natureof the glass ceramic with its composition varying on a submicron scalein the form of an embedded phase, typically in the form of nanoparticlesin a glassy matrix.

In particular, the phases of the glass ceramic can include the seeds, ahigh-quartz solid solution, and a substantially surrounding amorphousresidual glass phase. These phases have different dielectric constants.Thus, the glass ceramic includes interfaces where the dielectricconstant exhibits a jump discontinuity. As described above, theinteraction of high intensity laser radiation with the matter can leadto local plasma formation and thus to the formation of the describeddefects. Decisive for the type and strength of the interaction or forthe modification of the material associated therewith is the amplitudeof the electric field strength of the laser radiation, which isproportional to the root of the intensity. If a laser pulse propagatesthrough a glass ceramic, the electromagnetic field (the wave) will havean effect on the glass ceramic.

In accordance with the continuity conditions of the electromagneticfield theory, a superelevation in field strength will occur at theinterfaces of the phases, due to the different dielectric constants. Themagnitude of the superelevation depends on the ratio of the twodielectric constants. Thus, the superelevation in field strength at theinterfaces of the phases can be referred to as an “amplificationfactor”. Therefore, it is desirable to have phases with the mostpossible different dielectric constants in order to provide forfacilitated plasma formation with a high amplification factor.

It has surprisingly been found in experiments that not only thedifference between the dielectric constants is decisive for an efficientmaterial modification, but also the size of the individual phases. Theinteraction of particle size and amplification factor has turned out tobe crucial in this case.

In addition to the above-mentioned three-phase glass ceramics,two-phase, four-phase, and other multiphase glass ceramics andmultiphase materials of other material classes are also eligible for themethod according to the invention. In particular, the material can be acomposite material, such as a polymer material with finely dispersedparticles, for example. Glass ceramics can be optical filters, known asfilter glass, and glass ceramics with zero thermal expansion.

It may be contemplated that the predetermination of the size of theparticle volume is based on the assumption that the extent of the effectof the interface between two phases for the formation of aplasma—referred to herein as “interfacial effect”—is proportional to theparticle volume. The predetermination of the particle volume size mayfurthermore be based on the assumption that the particles have aball-like, i.e. substantially spherical shape. In other words, it can beassumed with this assumption that the interfacial effect is proportionalto the third power of the particle diameter.

Furthermore, for the predetermination of the particle size it can beassumed that the interfacial effect is proportional to the ratio of thedielectric constants of the two phases minus one (see above). In theratio of the dielectric constants, the greater dielectric constant, forexample that of the surrounding residual glass phase, is in thenumerator, and the smaller dielectric constant, for example that of thehigh-quartz solid solution, is in the denominator. Plasma formation willalways occur when the electromagnetic wave enters the medium with thesmaller dielectric constant.

On basis of the three assumptions mentioned above, the followingrelation can preferably be used as a characteristic parameter IE for theinterfacial effect: (particle volume/nm³)·(|Δε_(r)|/ε_(r))>x withxϵ{500, 1000, 2000}, with ε_(r) being the dielectric constant of theglassy medium.

In the above relationship, the first factor is a measure of the particlevolume and the second factor is a measure of the jump in electric fieldstrength. In case of equal dielectric constants, this factor and thusalso the interfacial effect is zero. The particle diameter is specifiedin nanometers (nm), and the division by nm3 is only used fornondimensionalizing the parameter IE.

Due to the interfacial effect IE, damage, i.e., the formation ofmodifications, will already occur at rather low intensities in glassceramics. Such modifications can then even be of a higher degree, moreextended, more pronounced, larger or longer than in green glass.However, since not so much energy can be deposited per area, the defectsare less pronounced in glass ceramics. By contrast, in a green glasscorresponding to the glass ceramic, higher intensities are required, sothat the damage will be greater there than in glass ceramics.

It has further been found that in the case of filamentary beampropagation, there will usually be a higher number of refocusing cyclesin green glass. This can be explained by the interfacial effectdescribed above. Since green glass requires higher intensities of thelaser to ignite the plasma, the equilibrium state which determines thelength of the filament is not so stable and collapses more rapidly.However, if the pulse has still sufficient energy afterwards, it canrefocus.

It may further be contemplated to adjust the desired size or extent ofthe modifications, in particular the length in case of linearmodifications, by adjusting the number of partial pulses per burst, ifthe burst mode is used. Depending on the configuration of the laser, thepartial pulses of a burst may carry, in total, slightly more energy thana corresponding single pulse in single-shot mode. However, the smallerthe number of pulses per burst, the higher is typically the energy ofthe first pulse of a burst. The higher the energy of the first pulse ofthe burst, the greater is the potential extent of the modifications.With the number of further pulses, only the so-called defect categoryincreases. Defect category is a measure of the degree of damage to thetransparent material (the extent of the modification). The followingdefect categories can be distinguished: 0=no modifications can beidentified under 100 times of magnification; 1=thin channel; 2=thinchannel with locally thicker areas; 3=extended damage; 4=cracks,micro-explosions, and/or melting range.

In comparison to glass ceramic, green glass requires an higher pulseenergy in the first partial pulse, if the burst mode is used, in orderto obtain the same size or extent of a modification, in particular thesame length of a linear modification. By changing the number of partialpulses per burst, the energy of the first pulse can be modified, wherebyit is possible to adjust the size or extent of the modification.

It may further be provided that the median of particle sizes is 6 nm ormore, preferably 8 nm or more, more preferably 10 nm or more. It seemsto exist a critical particle size above which the interfacial effect isparticularly effective. Taking into account this critical particle size,the aforementioned median of the particle size may be advantageous.

Preferably, finely dispersed additional particles may be provided in theproduction of the workpiece material in order to make the process ofproducing the modification more efficient.

With the method according to the invention, which can in particular beapplied to glass ceramics produced from sheet glass, it is possible toachieve better cutting performance with lower laser power than in thecase of homogeneous glasses, which is associated with a financialadvantage. The upper limit of the material thickness that can beprocessed, which is given due to the configuration-related limitation ofthe pulse energy of laser sources, is higher for multiphase materialsthan for homogeneous glasses, which is advantageous for an applicationon very thick or multi-layered materials.

In the composite material according to the invention, the regions of thefirst phase preferably surround the regions of the at least one secondphase at least partially, i.e. substantially. Particularly preferably,the regions of the at least one second phase are completely embedded bythe first phase in the composite material of the invention.

In the composite material according to the invention, regions of the atleast one second phase can be spaced apart from one another. Oneembodiment of the composite material according to the invention isdistinguished by the fact that the regions of the at least one secondphase are substantially spherical. In another embodiment of thecomposite material according to the invention, the lengths of the linearmodifications range between 100 and 10,000 μm, preferably between 1000and 10,000, more preferably between 3000 and 10,000 μm.

Also, the ratio of the first and second dielectric constants(ε_(r1)/ε_(r2)) may be greater than or equal to 1.05, better greaterthan 1.1, preferably at least 1.3. In other words, this means that|ε_(r1)−ε_(r2)|/ε_(r2), elsewhere also referred to as |Δε_(r)|/ε_(r),can be greater than or equal to 0.05, better greater than 0.1,preferably at least 0.3.

It will be understood that the composite material according to theinvention can be composed of more than two phases. A plurality of secondphases of different dielectric constants can be embedded in a firstphase.

The composite material according to the invention preferably includes analigned arrangement of linear modifications so as to form a separationline.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 shows a laser processing device while processing a workpiece on aworkpiece table;

FIG. 2 shows a linear modification formed by defects in a glass ceramic;

FIG. 3 shows lengths of linear modifications which have been produced inglass ceramic and in the corresponding green glass, for comparisonreasons, using lenses of different focal lengths of the focusing opticalunit, as a function of the number of pulses per burst;

FIG. 4 schematically shows two intensity profiles along the central axisof rays focused in the glass ceramic;

FIGS. 5a and 5b show scanning electron micrographs of linearmodifications in a glass ceramic and in a borosilicate glass,respectively.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrates embodiments of the invention and such exemplifications arenot to be construed as limiting the scope of the invention in anymanner.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a laser processing device 1 above a workpiece 2 whichis supported on a workpiece table 3. Laser processing device 1 comprisesan ultrashort pulse laser 10 and a focusing optical unit 11 in order toemit a focused beam of rays 12 with a focus 13 that is locatedapproximately on the upper face of workpiece 2. On workpiece 2, acutting line or fracture line 20 is indicated, along which the workpiece2 will be separated or divided. Provision is made for displacing thefocus 13 along this line 20, which can be accomplished by adjusting thetable 3 in the two coordinate directions 21, 22. Very small adjustmentincrements are used.

The wavelength of the radiation of ultrashort pulse laser 10 is selectedso that it is in a transparency range of workpiece 2. Ultrashort pulselaser 10 is operated in a so-called burst mode in which the actual pulseitself, referred to as a burst, is defined by a packet of pulsesrecurring at a repetition rate R of approximately 100 kHz. The energy ofthe laser pulses or of the pulse packets (bursts) is dimensioned so thatwith each burst a damage channel 14, referred to as a filament, isformed perpendicular to an upper face 2 a of workpiece 2 in the interiorthereof. By displacing the beam-shaping optical unit 11 along apredefinable displacement line 20, a series of linear modifications 14is generated in the workpiece 2. With an advancement rate of 1 m/s, thestarting points of damage channels 14 which are directed towards thebottom face 2 b, will have a spacing of 10 μm on the upper face 2 aalong displacement line 20 of the ultrashort pulse laser 10. Thedisplacement line 20 of ultrashort pulse laser 10 relative to workpiece2 defines a fracture face and is therefore referred to as a fractureline 20.

Such linear modifications 14 produced by the method of the invention andin the composite material of the invention are shown in FIG. 2. As canbe seen in FIG. 2, the linear modifications 14 are formed by defects 50,51, 52, and 53 with lengths of 108 μm, 221 μm, 357 μm, and 1037 μm,which have been produced or which are arranged along a damage channel.The defects shown in FIG. 2 have a spacing 60, 61, and 62 of 263 μm, 195μm, and 34 μm, respectively. Mean values from three measurementsequivalent to FIG. 2 yielded defect lengths of 104 μm, 237 μm, 350 μm,and 1020 μm with a spacing of 290 μm, 209 μm, and 150 μm.

This exemplary embodiment, which is limited to the generation of linearmodifications, shows that the defect lengths do not have to beconsistent. Later, this can be of great advantage in order to be able toproperly divide the workpiece along the alignment line of the filaments.

Accordingly, in a further embodiment of the method of the invention itis generally contemplated, without being limited to the specificexemplary embodiment explained above, that linear modifications 14 whichare aligned along a line 20 in a transparent workpiece 2, are eachformed of a plurality of defects that are arranged along a channel, andthat the lengths the defects increase with increasing distance from asurface of the workpiece. Thus, in case of a tensile or bending load, asit is applied to the alignment line of the linear modifications forintentionally breaking the workpiece, there will be shorter defects nearthe surface, that is where the greater bending stress occurs. In thevolume, towards the center of the workpiece, the tensile stress will besmaller in the case of a bending load. However, the defects are longerand the workpiece is weakened here to a greater extent. Thus, when theworkpiece is subjected to a bending load, the critical bending stress isdistributed more evenly along the filament structure, so that thefracture behavior is improved.

In the following, the investigation of a corresponding pair of glassceramic and green glass will be described, which are referred to asglass ceramic A and green glass A. Green glass A has a glass compositionthat contains Li₂O (lithium oxide), Al₂O₃ (aluminum oxide), SiO₂(silicon dioxide) and a total of about four weight percent of TiO₂(titanium dioxide) and ZrO₂ (zirconium dioxide). The transformationtemperature of this composition is 670° C.

On basis of this green glass A, the glass ceramic A has been produced byceramization, during which finely dispersed individual crystals arebeing formed in the glassy material. For this purpose, the green glass Awas processed in an electrically heated ceramization furnace. Theresulting transformation process of the material can be subdivided intonucleation and crystal growth. At the beginning of each crystallizationis a crystallization seed at which the crystal can begin to grow.

Green glass A contains impurities in the form of the added titaniumdioxide and zirconium dioxide, which have a high melting temperature(1855° C. and 2715° C., respectively), and precipitate when heated so asto be effective as crystallization seeds. They represent heterogeneousseeds during the ceramization process, whereby a high seed density and asmall crystal size is achieved. Due to crystal growth, a high-quartzsolid solution (HQ_(ss)) grows on the crystallization seeds oforthorhombically arranged ZrTiO₄. It is based on the LAS system whichtakes its name from the crystal building blocks of lithium oxide,aluminum oxide, and silicon dioxide. Quartz (SiO₂) transforms intoso-called high-quartz at 573° C. However, due to the incorporation ofother atoms, the HQ_(ss) is stable when this temperature is undershot.

An ultrashort pulse laser was employed for the investigations, using awavelength of 1064 nm, an average power of 12 W (at 1064 nm, 100 kHz, 1pulse per burst), a repetition rate of 100 kHz, a burst frequency of 50MHz, a pulse duration of about 10 ps (at 1064 nm and 100 kHz), and aGaussian beam as the beam profile.

The energy that is introduced and absorbed by a laser pulse leads to astronger heating in the glass ceramic A, due to the lower heat capacitycompared to green glass, and the developed heat is also dissipated moreefficiently than in the green glass A. However, in very short timeregimes in the nanosecond range, no significant heat dissipation is tobe expected, wherefore the assumption of a higher temperature in glassceramic A compared to green glass A is justified.

In the burst mode, the second pulse of the burst will therefore impingeat an already preheated area. The burst frequency of 50 MHz results inan interval of only 20 ns between the individual pulses, which satisfiesthe condition mentioned in the previous paragraph. Although the effectsof the burst mode in the USP laser blast technology have not yet beendefinitively clarified, yet there is a stronger presumption on the heatdeposition which influences the behavior of the further pulses. Theproduced defects are formed more efficiently with increasing number a ofpulses.

Therefore, a series of tests was carried out, in which linearmodifications were introduced and both materials were each processedwith different numbers of pulses a per burst, according to a preferredembodiment of the invention. Four lenses of different focal lengths wereused, in order to obtain a broader variety of data. According to thetheory outlined above, the ratio of the total lengths of the defects orlinear modifications in glass ceramic A to the corresponding totallength in green glass A should increase with an increasing number ofpulses per burst, since the effect of heat deposition becomes moresignificant with increasing number of partial pulses.

However, the energy distribution to the individual partial pulses of theburst must be taken into account. With only one partial pulse, the pulseenergy is 120 μJ and the peak pulse power is 12 MW. In case of thetwo-pulse burst mode, on the other hand, the first pulse has about 77 μJand the second 56 μJ. Therefore, if the number of pulses is high, thepulse energies and thus also the pulse peak powers and intensities maybe too low to cause a modification in the material.

The repetition rate, the advancement rate in y-direction (1 m/s), andthe power level (100%) were kept constant during the test series. FIG. 3shows the results.

No damage to the material occurred only with two constellations: greenglass A, 10 mm focal length, and four or eight pulses per burst,respectively. Here, the pulse energy of the first partial pulse of theburst was not sufficient to produce a modification of the material. Forother optics, this absence was not observed, since the beam shapegenerated by such other optics is probably more suitably.

The results show a strong dependence of the total length on the number aof partial pulses. With one exception (green glass A, focal length equalto 80 mm, and a equal to two), the total length decreases withincreasing number of partial pulses, as shown in FIG. 3, in which themeasurement points were connected in order to illustrate themonotonically decreasing characteristic of the measured data. This ispresumably due to the low pulse energy of the first pulse of theavailable laser, which decreases for increasing partial pulse numbers a.The assumption that the energy of the first pulse is responsible for thelength and the further partial pulses only amplify the type of damagehas thus been confirmed at least in the first point.

In the following, the investigation of a material that is used forproducing optical filters will furthermore be described. This is thematerial RG610, and this material was investigated as green glass(referred to as green glass RG610 below), and also as so-called filterglass produced by thermal treatment of the green glass and including atleast 2 different glassy or glassy-crystalline phases.

In the state of the filter glass that exhibits two phases, therefractive index is 1.52. The yellowish green glass RG610 changes colorby the heat treatment, whereby the filter glass RG610 appears deep redlater. The reason for this are the resulting mixed phases (amorphous orcrystalline) which contain cadmium, sulfur, selenium, and zinc. Sincethere is no refractive index available for these mixed phases, it wasderived from the refractive indices at the D-line of 588 nm of similarcrystals: zinc selenide, zinc sulfide, cadmium selenide, cadmiumsulfide. Thus, a refractive index of the crystal of 2.53 is resulting.Since the crystalline phase only accounts for a small volume fraction(about 12%), the refractive index of the residual glass phase can beapproximated by that of the green glass RG610 of 1.52. This results in aratio of the dielectric constants (ratio of the squared refractiveindices) of 2.770. The size of the crystals (diameter) is about 12 nm.

By applying the ultrashort pulse laser technology to RG610, thediscrepancy between green glass and glass ceramic could be clearlydemonstrated: The defects in the glass ceramic RG610 are more than twiceas long as with the green glass RG610 and reach a length of up to 5 mmin glass of 6.6 mm thickness.

TABLE 1 Particle Ratio of di- Interfacial Material diameter/nm electricconstants effect IE Filter glass RG610 12 2.770 3059 Glass ceramic A 401.107 6848 Pre-seeded material A 4 2.374 88

Table 1 shows the factors of the interfacial effect IE for RG610 (asfilter glass), and glass ceramic A (residual glass-phase and high-quartzsolid solution interface), and pre-seeded material A (residualglass-phase and seed interface), which were calculated using the abovestated formula.

Although the calculated value for glass ceramic A is more than twice aslarge as the value for the filter glass RG610 A, both values are of thesame order of magnitude. This is different for the value of thepre-seeded material A.

The green glass and the glass ceramic A is a lithium aluminosilicateglass of the composition:

-   -   60-73.0 wt % of SiO₂;    -   15-25.0 wt % of Al₂O₃;    -   2.2-5.0 wt % of Li₂O;    -   0-5.0 wt % of CaO+SrO+BaO;    -   0-5.0 wt % of TiO₂;    -   0-5.0 wt % of ZrO₂;    -   0-4.0 wt % of ZnO;    -   0-3.0 wt % of Sb₂O₃;    -   0-3.0 wt % of MgO;    -   0-3.0 wt % of SnO₂;    -   0-9.0 wt % of P₂O₅;    -   0-1.5 wt % of As₂O₃;    -   0-1.2 wt % of Na₂O+K₂O, with respective amounts within the        following specified ranges:    -   0-1.0 wt % of Na₂O;    -   0-0.5 wt % of K₂O; and    -   0-1.0 wt % of coloring oxides.

For two lenses it has been shown above that with a parameter adjustmentit is possible to achieve the same filament structure length in glassceramic A as in green glass A. In this case, it is also possible toachieve the same defect category as in green glass A. Accordingly, theenergy of the first partial pulse of the burst is decisive for thedefect length, whereas it has been found that the further pulses onlyincrease the defect category. With the same parameters, the defects inglass ceramic A will in fact be longer than in green glass A, but with alower defect category. With an increased number of partial pulses perburst, the defects become shorter and the defect category becomeshigher. It was found that with eight partial pulses, the same resultscan be achieved in glass ceramic A as with two partial pulses in greenglass A.

Due to the interfacial effect IE in glass ceramic A, damage is alreadycaused at lower intensities, so that the defects are longer than ingreen glass A. However, since not so much energy can be deposited perarea, the defects are not so pronounced. Green glass A, on the otherhand, requires higher intensities, so that the damage is more pronouncedhere. This can be compensated with an increase in the number of partialpulses per burst, as mentioned above.

The usually higher number of refocusing cycles for green glass can alsobe explained with this model. Since higher intensities of the laser arenecessary in case of green glass to ignite the plasma, the state ofequilibrium which determines the defect length is more unstable andcollapses more rapidly. However, if the pulse has still sufficientenergy afterwards, it can refocus.

For producing the material modification, a power threshold value isreached or exceeded. Due to the described effect of field strengthsuperelevation, this threshold value will usually be significantlysmaller for glass ceramics than the value for the corresponding rawglass (see the table, all values in W/m²):

Material 8712 8724 state raw glass 1.0*1E16 1.5*1E16 ceramized 0.4*1E160.6*1E16

In the case of not optically optimized setups, e.g. when the laserradiation is focused using a lens with spherical aberration, asignificantly higher intensity and power occurs in the material at therear end of the focusing area (in beam direction) than in the vicinityof the imaging optics, so that the threshold values are substantiallyonly exceeded in the vicinity of the intensity peaks. With adaptedoptics, however, the same pulse energy (equal areas under the curves) isdistributed more homogeneously in the focus area, ideally ashomogeneously as possible above the glass ceramic threshold, so that asa result of the field strength amplification effect described above thematerial modification can be produced over a significantly longer regionthan in glass.

This effect is illustrated below with reference to FIG. 4.

FIG. 4 schematically shows two curves J1, J2 of the power per volumealong the central axis of the propagation direction of laser beamsfocused in the glass ceramic. The values x along the abscissa indicatethe distance to the lens. The desired linear modifications are producedabove certain power per volume thresholds. In FIG. 4, a threshold value(P/V)_(min,glass) is indicated for producing the modification in glass,and a lower threshold value (P/V)_(min,glass) for producingcorresponding modifications in glass ceramics.

Intensity profile J1 results when a lens with spherical aberration isused for focusing, for example.

Intensity profile J2 is an optimized profile as it can be achieved usingan axicon, for example. With the focusing, an elongated intensitymaximum is achieved. With the same pulse energy, the peak intensity willthen be correspondingly lower than in the case of intensity profile J1.

The length of the produced linear modifications now depends on thelength of the region in which the pulse power per volume exceeds therespective threshold value. Intensity profile J1 still allows for alinear modification of a length Lglass in glass. However, with intensityprofile J2, no modification can be achieved, since the threshold value(P/V)_(min,glass) is no longer exceeded with the comparatively lowermaximum intensity. In glass ceramics, on the other hand, this is notonly possible with the lower threshold value (P/V)_(min,glass) ceramic,but moreover the achievable length L_(glass ceramic) is longer. Becausethe focal range in which the threshold value is exceeded is elongated inthe direction of the beam, a correspondingly elongated linearmodification is achieved. Accordingly, the length of the linearmodification can be maximized by a preferably homogenous distribution ofthe laser pulse energy and power per unit volume above the minimumenergy and minimum power per unit volume along the focal line.

In order to achieve a longest possible length of the modification, anoptical systems may therefore be selected for focusing the laser beamdepending on the material of workpiece 2 and at a given pulse energy,which generates an elongated focus with a reduced maximum intensitycompared to a spherical lens, and so that the threshold value forproducing the material modification will not be undershot. In otherwords, an optical unit is therefore used for focusing the laserradiation, which spatially extends the focus in the propagationdirection, so that the maximum intensity of the laser radiation is lessthan 150%, preferably less than 130% of the intensity threshold valueabove which the material is modified.

A suitably intensity distribution is in particular achieved when thebeam is focused using the optical unit so that the ratio of the lengthrange along which the intensity is at least 110% of the threshold valueof the modification to the length range along which the intensity is atleast 10% of the threshold value is at least 0.4, preferably at least0.5, more preferably at least 0.7. In this way, a particularly suitabledistribution of the intensity is achieved so that a very elongatedregion in the material is irradiated with intensities above thethreshold value.

From FIG. 4 it is moreover apparent that the length of the modificationcan be easily adjusted via the pulse power. With increasing power, thelength section is extended along which the threshold value for producingthe modification is exceeded.

FIG. 5a shows an SEM image of a plurality of adjacently arranged (fromleft to right in the figure) linear modifications 14 (and extending fromtop to bottom in the figure) which are produced by the method accordingto the invention and in the composite material according to theinvention.

The nine visible linear modifications 14 are spaced apart from eachother by about 7 micrometers and were produced by laser pulses in theform of bursts and with a biconvex lens (16 mm) using the followingprocess parameters: 6 bursts, burst frequency: 50 MHz, total burstenergy: about 500 μJ, decreasing burst shape, pulse energy of the firstpulse about 170 μJ, 12 mm tube beam 1/e², wavelength: 1064 nm.

It can be seen in FIG. 5a that the linear modifications aresubstantially wider than for example in a borosilicate glass which isshown in FIG. 5b , for comparison reasons, and which has filamentarychannels with diameters in the sub-micrometer range.

A linear modification or a defect of a linear modification canaccordingly have a width, in particular an average width 70, which isgreater than 1 μm, preferably greater than 2 μm. “Width” herein means adimension perpendicular to the extension of the linear modification.

Such a relatively wide linear modification or defect of a linearmodification may in particular exist in the form of a melting zone.Accordingly, material of the composite material may melt around theimpact region of the laser and may undergo a phase transformation, forexample.

In addition to the relatively wide impact zones, it can also be seenfrom the SEM image of FIG. 5a that the linear modifications areaccompanied by bubble formation in certain regions along their extensiondirection (along the laser beam axis). The bubbles may in particular bevoids, for example at the phase boundaries, due to the field strengthsuperelevation which was already described above. It can also happenthat an additional filament channel is formed along the linearmodifications.

Accordingly, a linear modification may include at least partially openareas 80, in particular pore-like or bubble-shaped areas.

While this invention has been described with respect to at least oneembodiment, the present invention can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

What is claimed is:
 1. A method for producing modifications in or on atransparent workpiece utilizing a laser processing device, wherein thelaser processing device has a short pulse or ultrashort pulse laser thatemits laser radiation having a wavelength in the transparency range ofthe workpiece and which has a beam-shaping optical unit for beam shapingfor focusing the laser radiation; and wherein the transparent workpieceis composed of a material that has a plurality of phases, of which atleast two phases have different dielectric constants, of which in turnthe one phase is a phase embedded in the form of particles, which phaseis substantially surrounded by the other phase, and wherein the productof the volume of the particles specified in cubic nanometers and theratio of the absolute value of the difference of the two differentdielectric constants to the dielectric constant of the surrounding phaseis greater than
 500. 2. The method as claimed in claim 1, wherein theproduct of the volume of the particles specified in cubic nanometers andthe ratio of the absolute value of the difference of the two differentdielectric constants to the dielectric constant of the surrounding phaseis greater than
 1000. 3. The method as claimed in claim 1, wherein theproduct of the volume of the particles specified in cubic nanometers andthe ratio of the absolute value of the difference of the two differentdielectric constants to the dielectric constant of the surrounding phaseis greater than
 2000. 4. The method as claimed in claim 1, wherein aseries of adjacent linear modifications is generated in the transparentworkpiece along a line by moving the laser radiation relative to theworkpiece along said line and by emitting laser pulses in timesuccession, either as individual pulses or in the form of bursts,wherein each of said individual pulses or bursts is used to produce arespective one of said linear modifications.
 5. The method as claimed inclaim 4, wherein the length of the linear modification is adjusted viathe pulse energy level.
 6. The method as claimed in claim 4, whereinsaid linear modification is generated by beam shaping with formation ofa line focus or by beam shaping with formation of a sufficiently highintensity for a modification along the optical axis.
 7. The method asclaimed in claim 1, wherein an optical unit is used for focusing thelaser radiation, which spatially extends the focus in the propagationdirection in such a way that the maximum intensity of the laserradiation is less than 150% of the threshold value of the intensityabove which the material is modified.
 8. The method as claimed in claim7, wherein the maximum intensity of the laser radiation is 130% of thethreshold value of the intensity above which the material is modified.9. The method as claimed in claim 1, wherein the laser radiation isfocused so that a ratio of the length range along which the intensity isat least 110% of the threshold value of the modification to the lengthrange along which the intensity is at least 10% of the threshold valueis at least 0.4.
 10. The method as claimed in claim 9, wherein the ratioof the length range along which the intensity is at least 110% of thethreshold value of the modification to the length range along which theintensity is at least 10% of the threshold value is at least 0.5. 11.The method as claimed in claim 9, wherein the ratio of the length rangealong which the intensity is at least 110% of the threshold value of themodification to the length range along which the intensity is at least10% of the threshold value is at least 0.7.
 12. The method as claimed inclaim 4, wherein the linear modification is generated by a beam shapingwith a rectilinear or curvilinear high intensity distribution.
 13. Themethod as claimed in claim 4, which comprises providing a workpiecetable and a displacement device for aligning the beam-shaping opticalunit to the workpiece for producing said linear modification in theworkpiece and thereafter for an incremental relative displacementbetween the beam-shaping optical unit and the workpiece table accordingto the line.
 14. The method as claimed in claim 4, wherein the linearmodifications are introduced in the workpiece in a predetermineddirection, wherein said direction is controlled with respect to thelocal surface normal of the workpiece and at a distance to theworkpiece.
 15. The method as claimed in claim 4, wherein the linearmodifications are each formed of a plurality of defects arranged along achannel, wherein the lengths of the defects increase with increasingdistance from a surface of the workpiece.
 16. The method as claimed inclaim 1, wherein modifications are generated at the surface of thetransparent workpiece by removing material from the surface by saidlaser radiation.
 17. The method as claimed in claim 1, whereinmodifications are produced inside the transparent workpiece by adjustingthe parameters of the short pulse or ultrashort pulse laser in such away that damages to the surface of the transparent workpiece areavoided.
 18. The method as claimed in claim 1, wherein the embeddedphase is defined by at least one particle.
 19. The method as claimed inclaim 1, wherein said material is a glass ceramic or a compositematerial.
 20. The method as claimed in claim 1, wherein a material isused whose particles have a median of particle sizes of 6 nm or more.21. The method as claimed in claim 1, wherein an optical element is usedto deflect the laser light and to successively modify the point ofincidence of the laser beam.